Process for pretreating steel plates for enamelling

ABSTRACT

A PROCESS OF PENETRATING STEEL PLATES FOR ENAMELLING BY DEGREASING, PICKLING, METALLISING, NEUTRALISING AND DRYING, WHEREIN METALLISING IS CARRIED OUT GALVANICALLY IN AN ACID BASE WHICH CONTAINS BESIDES NICKEL AND/OR COBALT IONS MAGNESIUM AND CHLORINE IONS, THE NICKEL AND COLBALT IS APPLIED TO THE SURFACE TO BE ENAMELLED IN QUANTITIES OF FROM 0.3 UP TO 0.7 G./M.2, CALCULATED AS METAL. OPTIONALLY, BEFORE THE METALLISING STEP AN ELECTROLYTICALLY DEGREASING AND PICKLING STEP IS PERFORMED. BY THE AFORESAID TREATMENT THE OVERALL TIME CONSUMPTION OF PRETREATMENT CONSIDERABLY SHORTENED. THE PRETREATING IS SUITABLE FOR STEEL SURFACES TO BE ENAMELLED AND IS PARTICULAR ADVANTAGE WITH DIRECT ENAMELLING PROCESSESS.

United States Patent US. Cl. 204-34 7 Claims ABSTRACT OF THE DISCLOSURE A process of pretreating steel plates for enamelling by degreasing, pickling, metallising, neutralising and drying, wherein metallising is carried out galvanically in an acid base which contains besides nickel and/or cobalt ions magnesium and chlorine ions; the nickel and cobalt is applied to the surface to be enamelled in quantities of from 0.3 up to 0.7 g./m. calculated as metal. Optionally, before the metallising step an electrolytically degreasing and pickling step is performed. By the aforesaid treatment the overall time consumption for pretreating is considerably shortened. The pretreating is suitable for steel surfaces to be enamelled and is or" particular advantage with direct enamelling processes.

This application is a continuation of Ser. No. 864,516, filed Oct. 7, 1969, now abandoned.

In conventional enamelling, nickeland cobalt-containing ground coats are used to promote adhesion between the surface or the steel and the enamel. Recently eiforts have been made to replace two-layer enamelling by onelayer enamelling. In one-layer enamelling, also known as direct enamelling, it is also possible to apply and to fire white or light-coloured enamels directly onto the steel surface. In this case, adhesion of ,the layer of enamel is governed by the pretreatment of the steel surface; in other words, the pretreatment takes over the function of the ground coat in promoting adhesion.

In contrast to conventional enamelling, degreasing is followed in the direct enamelling process by an intensive pickling treatment whose function is to cause etching (pickling loss) in a quantity of from 20 to 40 g. of metal per square metre of surface, depending upon the quality of the steel. Pickling is in turn followed by a nickel plating in which from 1 to 1.5 g. of nickel per square metre are deposited. Degreasing is a particularly important stage in any enamelling process and especially in direct enamelling, being carried out in several stages. Depending on the state in which it is delivered, the steel plate also has to be derusted and then degreased. In conventional processes, the entire pretreatment, including the necessary rinsing baths, comprises at least eight and, in direct enamelling, at least eleven treatment stages. In most cases, however, the number of stages is even exceeded. In addition to the aforementioned treatment stages, neutralising and drying are usually necessary to complete the process. After drying, the enamel frits are applied to the plate in the usual way and after further drying are fired.

In the pretreatment processes that are used in practice, the residence times in the individual stages amount to about eight minutes in cases where the immersion or dip process is used. Degre'asing can even take two to four times longer. Accordingly, time consumption is considerice able for the completion of an average of seven to twelve pretreatment stages.

In direct enamelling, there is no reduction in time even when pretreatment is carried out by the spray process. It is only in conventional enamelling that the overall working time can be reduced by about half using the spray process. In dip processes, the residence times in the currentless nickel baths by which pickling is followed are determined by the reaction velocities of the exchange or reduction nickel-plating processes, and cannot be shortened to any bearing extent by increasing the temperature. There have been many attempts to replace the exchange or reduction nickel-plating process by a galvanic nickel-plating process. Galvanic nickel-plating can be carried out not only more quickly but also more economically. On the one hand there is no need for any nickel salts for regeneration and on the other hand the accurate bath checks and the apparatus for bath regeneration so necessary in conventional processes are redundant. Under the electrical conditions applied, only as much nickel is removed from the nickel anodes as is deposited onto the steel surfaces to be pretreated.

However, it has hitherto been impossible to utilise the aforementioned advantages of galvanic nickel-plating in enamelling because the adhesion obtained was always inadequate. It was found that when applied in a layer of adequate thickness, the enamel was unable to reach the plate with the result that the bond-forming reactions could not take place (Dietzel Metalloberfl'alche 12 (1967), p. 372).

A process of pretreating steel plate for enamelling by degreasing, pickling, metallising, neutralising and drying has now been found, in which metallisation is carried out galvanically, in an acid magnesium ions containing bath with a nickeland/or cobalt-salt-content of between about 8 and 30 g./l., calculated as metal, nickel and/or cobalt being deposited onto the surfaces to be enamelled in quantities of from about 0.3 to 0.7 g. per square metre, expressed as metal.

The galvanically deposited layer of nickel is suitable for both the conventional enamelling processes and the direct enamelling processes. It is also surprising that, instead of or in addition to the nickel, cobalt can also be deposited in a form which ensures firm adhesion of enamel. Hitherto, cobalt has only been used in the form of its oxide in the ground coat as a binder, and satisfactory metallic deposition in pretreatment baths has not been possible. It is known that cobalt is a much more effective component than nickel in ground coat enamels. Accordingly, the process according to the invention enables the enameller optionally to deposit nickel and/or cobalt layers in order to obtain optimum adhesion between the enamel and the substrate according to the prevailing conditions. In addition, to this the galvanically deposited nickel and/or cobalt layers provide the layer of enamel with an excellent surface finish. No special steps have to be taken to carry out the process according to the invention. The containers used in conventional nickel-plating processes, for example, of plastics such as polyvinyl chloride or polypropylene, or even rubber coated steel containers, may be used as the tanks. There is no need for the temperature of the baths to be regulated because deposition takes place sufliciently quickly at temperatures of from about 15 to 25 C. Although lower or higher temperatures can be used, they are not economic.

The plate to be pretreated is connected as the cathode in the usual way, whilst pure nickel or cobalt or their alloys are used as anodes. Normally, the bath is operated at average current densities of from about 0.3 to 0.8 amps/dm. although it may also be operated at higher current densities in special cases where particularly quick deposition is required. In cases such as these, appropriate measures must be taken to ensure that the workpieces are not burnt or scorched in any way. It is best to use either moving electrolytes of moving electrodes.

The baths are best prepared with water-soluble nickel and/or cobalt salts, sulphates and/or chlorides in particular. The concentration expressed as metal should amount to between about 8 and 30 g. per litre of bath liquid. It is best not to add any complex formers to the baths, although it can be of advantage to add boric acid or other butler substances in such quantities that the pH value of the baths does not exceed a value of 7. In general, the baths can be operated at pH values of from 7 to 1, pH values of from 3.5 to 6 being preferred. Most of the metal should be present in substrate form. To accelerate the solubility of the anodes, chlorides are with advantage added to the baths, preferably in quantities of from 3 to 20 g. of Cl/litre. The chloride ions can be introduced in the form of metal chlorides or as alkali metal chlorides, including ammonium chlorides. Deposition of the nickel and/or cobalt is further promoted by the presence in the baths of magnesium ions. Very small quantities of from 0.1 to 10 g. Mg. per litre are sufficient. Larger quantities are not harmful because the magnesium is not deposited and only indirectly affects the form in which the nickel and/or cobalt is deposited along the required lines. The magnesium is introduced in the form of the water-soluble salts, preferably in sulphate and/or chloride form.

The baths can be used for an almost unlimited period providing impurities are not entrained into them by the workpieces introduced. There is no need for regeneration as the nickel or cobalt concentration is automatically adjusted to the required level because as much nickel or cobalt from the electrodes is dissolved in the bath as is deposited on to the metal surfaces to be treated.

It is of course also possible to use insoluble anodes, for example of platinum, titanium, graphite and so on, but only at the expense of one advantage of the process according to the invention over conventional processes, because the metal salt concentration not only has to be replenished during operation but also has to be checked.

Compared with conventional metallising baths, therefore, there is no need for temperature control, replenishment or regeneration. Another important factor is that the amount of time required to deposit adequate metal layers is much shorter than in conventional processes. Residence times as short as from 10 to 120 seconds are required.

Following metallisation, there is no need to neutralise the plates. All that has to be done is to rinse away any salt residues still adhering to the surface.

The process according to the invention is not only suitable for pretreatments completely carried out by the dip process. The electrolytic dip-metallising process affords special advantages when combined with processes in which the remaining pretreatment stages are carried out in spraying machines. In this case, the basically short pretreatment times in the spraying machines can be maintained or made even shorter in the metallisation stage also. Accordingly, the process can readily be incorporated into already existing spraying plants so that these too can readily be used for direct enamelling.

The process according to the invention is suitable for any type of enamelling. In conventional enamelling, i.e. in cases where ground coat enamels are used, the quantity of oxide binders in the frits can be considerably reduced. lt is a major advantage of the process according to the invention that the absolute quantity of binders in the form of nickel and/or cobalt can be reduced because, through their deposition in metal form on to the objects to be enamelled, these adhesion promoters are able to act much more effectively. The process according to the invention is of particular advantage in cases where loW- melting enamels with firing temperatures below 760 C.

are used in which cases the effect of the fused oxide binders is considerably reduced in view of the high viscosities.

In addition, the process according to the invention can replace all hitherto described nickel-plating processes in pretreatment for direct enamelling. Both low-carbon and normal enamelling steels can be pretreated. The present nickel-plating process does not require any special enamel frits. All the hitherto known ground coating and covering or surfacing enamels, including in particular borontitanium white enamel, are suitable, governed of course by the usual limitations in terms of materials and processing techniques. It has also been found that it can be advisable in some cases to carry out a thin-phosphate treatments after the usual pickling treatment and before galvanic nickel plating. For the thin-phosphating treatment, the pickled and rinsed plates are immersed in a solution which contains non-layer forming phosphates, preferably magnesium phosphate. It has proved to be of particular advantage to use I to 5% solutions of magnesium dihydrogenphosphate. The phosphating treatment is able to correct differences in the behaviour of dilferent types of steel.

Furthermore, it has been found advantageously to perform a electrolytically pickling and optionally also an electrolytically degreasing before the galvanic metallisa tion. The pickling is optionally performed with current densities of between 2 and 20 a./dm. in acid sulphate and/or phosphate containing solution at temperatures of between 40 and C. Normally a pickling time of 3 minutes is sufficient with a pickling loss of about 20 g./l. of iron per 111. with decarburised steel. The SO -ion concentration, expressed as sulphate should amount to between 2 and 25% by weight, and the PO -ion concentration, expressed as phosphate, to between 2 and 20% by weight. The sulphate ions can be introduced in the form of sulphuric acid, alkali metal sulphates and/or alkali metal bi-sulphates. In addition to the alkalins, preferably sodium and/or potassium, it is also possible to use magnesium sulphate or the bi-sulphate of magnesium. The PO -ions can be present in the form of phosphoric acid, alkali metal phosphates, magnesium phosphates, alkali metal phosphates, alkali metal dihydrogen phosphates, magnesium hydrogen phosphate and/or magnesium dihydrogen phosphate. It has been proved to be particularly suitable to use a bath containing about 1 to 5% by weight of magnesium dihydrogen phosphate. Phosphate-containing baths are generally of advantage because they have a favourable effect upon the subsequent metallising stage, especially the nickel-plating of the plates to be enamelled. Phosphate-containing baths have the further advantage that the iron which is dissolved during pickling is cathodically deposited. The service life of the baths is considerably lengthened in this way.

The process is suitable for both low-carbon and normal steels. In either case, firmly adhering direct enamel finishes are obtained by the pickling process according to the invention in conjunction with the other necessary pretreatment stages.

In addition to the reduction in working time and the saving of material through the smaller degree of etching, the longer service life of the baths affords another advantage, i.e. it enables some of the pressure to be taken off the effluent purification plants.

The reduction in working time achieved through this electrolytic pickling can with advantage be increased even further by an electrolytic degreasing stage.

A further improvement in the process is obtained by combining the electrolytic pickling according to the invention, and, optionally, the electrolytic degreasing stage with a galvanic metallisation stage following the pickling stage. By combining the electrolytic pickling with the electrolytic metallising and, optionally, electrolytic degreasing, the pretreatment time is considerably shortened in comparison with conventional processes. In many cases, variations in quality in the types of steel can in addition be corrected by a completely electrolytic pretreatment of the plates, so that firm adhesion is guaranteed in the subsequent direct enamelling operation. In addition to outstanding adhesion, a satisfactory enamel surface is also obtained in the same way.

By virtue of the short residence times in the individual treatment stages, it is possible without any loss in capacity also to take relatively small individual components continuously through the various treatment stages by the chain method. Accordingly, there is no longer any need in many cases to fill and to empty baskets or the like with the objects to be enamelled. More particularly, it is also possible to provide an electrostatic or electrophoretic deposition plant for the enamel frits immediately after a fully electrolytic pretreatment system. In this way, it is possible providing the individual operations are suitably arranged to make fabrication between the plate-forming stage and the enamelling stage substantially continuous.

The process according to the invention is illustrated by the following examples:

EXAMPLE 1 A cold-rolled steel plate of the following approximate analysis: C=0.08, Mn=0.32, S=0.035, P=0.05,

Si=0.02, Cu=0.l0, was subjected to alkaline degreasing, rinsed first hot and then cold, pickled in 9% sulphuric acid, rinsed and finally neutralized and dried. A primer enamel of the following composition: 48% of SiO 20% of B \18% of alkali metal oxides, 3% of fluoride, 5% of ZrO 2% of MnO 0.03% of C00, 0.7% of NiO, was then applied in the usual way to the plate thus pretreated and fired at a temperature of 820" C. Since the oxide binder content of the frit was very low, the adhesion of the enamel coating as determined with a needle tester (Erichsen indentation) amounted to only 50%. To achieve adhesion, the oxide binder contents had to be increased for the described pretreatment in the frit of the above composition from 0.3% of C00 of 0.6% of C00 and from 0.7% of NiO to 1.5% of NiO. If, however, pickling was followed by treatment with a galvanic metallising bath according to the invention of the following composition: CoSO -7H O 30 g./litre, (NH SO 60 g./litre, NiSO -7H O 30 g./litre, NH Cl g./litre, H BO anhydrous 12 g./litre, MgSO -7H O 12 g./litre, the remainder being water made up to 1 litre and binder metal was applied to the metal surface with this bath in a quantity of 0.2 to 0.3 g. of metal per square metre of surface at a current density of 0.45 a./dm. it was possible to obtain 100% adhesion even with the frit of low oxide binder content.

Proceeding on the assumption that some 500 g. of ground coat enamel were applied per square metre of plate and that the mass consisted of approximately 81.5% of enamel, 6.5% of clay and the rest quartz or feldspar, it is possible to calculate a savingof metal binder of more than 50% in cases where metallising is carried out in accordance with the invention.

EXAMPLE 2 A steel plate of the above analysis was subjected to alkaline degreasing, rinsed first hot and then cold, pickled in 9% sulphuric acid and then electrolytically degreased in accordance with the invention. In a bath of the composition: 80 g. of NiSO -7H O, 10 g. of NH Cl, 20 g. of MgSO -7H O, 5 g. of anhydrous H BO the remainder being water made up to 1 litre, nickel was deposited in a quantity of approximately 0.3 g. of Ni/sq. metre of surface at a pH value of from 5.3 to 5.7 and at a current density of 0.5 a./dm. After the pretreatment, the plates were coated in the usual way with an NiO- free primer enamel of the composition: 38.7% of SiO 7% of A1 0 22% of B 0 25% of alkali metal oxides,

6 6% of tfluoride, 0.4% of C00 and 0.9% of CaO, and the enamel was fired at 720 C. Adhesion of the enamel coating was outstanding and did not show any sign of deterioration following the stoving of a surface enamel which is also carried out at 720 C.

EXAMPLE 3 A decarburisecl steel plate of the composition: C 0002-0003, Mn 0.32, P 0.015, S 0.025, Si 0.01, Cu 0.03, was pretreated in the manner usual for direct enamelling, subjected to alkaline degreasing, intensively pickled (erosion 25 g./m. of surface area) and then nickel-plated. Nickel plating was carried out in accordance with the invention, i.e. electrolytically in a bath of the composition 70 g. of NiSO -7H O, 15 g. of NH Cl, 12 g. of MgSO -7H O', 7 g. of anhydrous H BO the remainder being water made up to 1 litre at pH 5.5 and at a current density of 0.7 a./dm. The nickel was applied in a quantity of 0.5 g. of nickel per square metre of surface area. The plate was then directly coated with a boron-titanium white enamel of the composition 38% of Si0 0.5% of A1 0 21% of B 0 15% of alkali metal oxide, 2% of fluoride, 19% of TiO 0.5% of MgO, 3% of P 0 and the enamel was fired at 820 C. Both the adhesion and the finish of the direct enamel coating were outstanding.

Normal enamelling steels can also be similarly pretreated for direct enamelling providing special acid mixtures for example, 15 to 20% of H 1 to 3% of HNO and 2 to 4% of urea, are used instead of the 9% sulphuric acid.

EXAMPLE 4 Decarburised steel plates of the approximate analysis: C=0.002 to 0.003; Mn=0.32; S=0.035; P=0.05;

Si=0.01; Cu=0.08 are preheated in accordance with the following schedule.

Time, Time, Procedure according to minmininvention utes Conventional procedure utes Alkaline boil-degreasing 6 Alkaline boil-degreasing. 8 Electrolytic degreasing 2 do 8 Hot rinsing 2 Hot rinsing 2 Cold rinsing 2 Cold rinsing 2 Electrolytic degreasing 70 C do 2 150g. NazSO -10H2O/L,

5 mLHzSOi conc., Sulphuric acid pickling 8 H2SO4/L; 10 AJdm. 2 Cold rinsing 1 Cold rinsing 1 Galvanic nickel plating 0.5 min. at ca. 0.5 A./dm. ca. 0.4 g.Ni/m. surface area is applied to the 0. 5 Exchange nickel plating 8 ObJect 1.2% NiSO4 7 1120. Bath composition: 80 g. NiSO 7 H 0, 10 g. NH4C], 20 g. M soi. 7 H20, 5 g. H3BO3, resid. H2O to l 1.:

Cold rinsing 1 Acid rinsing 1 Hot rinsing 1 Cold rinsing 1 Ncutralisation- 5 Total 16. 5 Total 46 Hot-air dryer IOU-120 C Hot air dryer -120 O The plates pretreated by the two processes outlined above were coated with a boron-titanium white enamel frit of the composition 38.3% SiO 0.3% A1 0 21.8% B 0 15.3% alkali metal oxide; 2.0% fluoride, 20.4% TiO 0.2% MgO; 3.3% P 0 and then fired at around 800 C. In both cases, the result of enamelling in terms of adhesion and finish were outstanding.

We claim:

1. In a process of enamelling a steel surface comprising the steps of degreasing, pickling, metallizing, neutralizing, drying and enamelling said surface, the improvement comprising performing the metallizing galvanically in a bath having a pH in the range of 3.5 to 7 and containing:

(1) a salt of nickel or cobalt,

(2) magnesium ions in an amount in the range of 0.1

to 10 g. Mg/L, and

(3) chlorine ions in an amount in the range of 3 to g./l., and depositing about 0.3 to 0.7 g/m. of said nickel or cobalt on said surface.

2. The process of claim 1 wherein said metallizing is carried out at a bath temperature in the range of 15 to C., and said salt is present in an amount in the range of 8 to g./ 1., calculated as metal.

3. The process of claim 1 wherein said salt is a nickel salt and said surface is treated with a non-layer forming phosphate solution before said nickel is deposited.

4. The process of claim 3 wherein the pickling bath contains from about 1 to Weight percent of magnesium dihydrogen phosphate.

5. The process of claim 1 wherein said pickling is carried out electrolytically at a temperature of from 40 to C. and at a current density of from 2 to 20 a./dm. said surface to be pickled being pretreated for up to about three minutes in an acid sulfateor phosphate-containing solution having as the case may be SO -ion concentration in the range of from 2 to 25 weight percent, calculated as sulfuror a Po -ion concentration in the range of from 2 to 20 percent by weight, calculated as sodium phos phate.

6. The process of claim 5 wherein said sulfate ions are present in the form of sulfuric acid or in the form of the neutral and acid sulfates of the alkali metals or magnesium.

7. The process of claim 5 wherein the phosphate ions are present in the form of phosphoric acid or in the form of the neutral and acid phosphates of the alkali metals or of magnesium.

References Cited UNITED STATES PATENTS 1,975,818 10/1934 Work 204-49 2,748,066 5/1956 Whitehouse et al. 204-34 2,819,207 1/1958 Shepard 204-38 O OTHER REFERENCES APC SN233,325, May 1943, Beck.

FREDERICK C. EDMUNDSON, Primary Examiner US. Cl. X.R. 204-38 C, 48, 49

UNITED STATES PATENT ormcn CERTIFICATE OF CCREC'HON Patent No. ,753,870 Dated August 21, 1973 Inventor(s) Hans Hoffmann et 3.1,,

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 16, change "substrate" to sulphate Column 5, line 46, between "NH Cl" and "g/litre" insert 6 This certificate supersedes Certificate of Correction issued October 1, 1974.

Signed and sealed this 24th day of December 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR. C. I IARSHALL DANN Attesting Officer Commissioner of Patents FORM Po-wso (10-69) USCOMWDC 6037M? U.S4 GOVERNMENT PRINTING OFFICE I969 0-355-334,

mg? m s'm'nss r xiwrjum lcx-s,

- CERTIFICATE GF'GQRRECTION Patent No. 3 753 870 I Dated Au ust 21, 1973 Inventor) Hans Hoffman et a1 It is crtified that error. appears' in the above identifi l.

I ed atent and that said Letters Patent are hereby corrected as shown below? T0221. 3, 'l ine 16, change substrate w w sgwhate "a -Z' Signed'and sealed this 1st day of October 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR. C. MARSHALL DANN I Attesting Officer I Commissioner of Patents 

